Electric wave amplifying system



July 25, 1939.- s. T. MEYERS 2,167,353

- ELECTRIC vmvE AIPLIFYING SYSTEM J Original Filed Dec. 5, 1956 Q I 2Sheets-Sheet 1 {5' [L n, Z2 7 g )6 g [2 T I i 2 v 7, E E 6 INVENTOR 5.7TMEVERS 6. C ATTORNEY July 25, 1939. s. T. MEYERS' ELECTRIC WAVEAMPLIFYING SYSTEM 2 Sheets-Sheet 2 originalFiled Dec. 5, 1936 I FIG/2F/GZA /Nl EN7'OR By S. T ME YE R5 ATTORNEY Patented July 25, 1939 toBell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Original application December 5, 1936, SerialNo; 114,400. Divided and this application October 15, 1937, Serial No.169,116 v 2 Claims. (01. 179 171 This application is a division'of myprior application Serial No. 114,400, filed December 5, 1,936, .forElectric wave amplifying system. v t This invention relates to waveamplifying sysems.

Objects of the invention are to control transmission properties of suchsystems, as for example, transmission efficiency, distortion, feedback,and impedances, and relations of such properties insuch systems;

It is alsozan' object of the invention to provid simplesystemsafi'ording such control.

'In onenspecific aspect the invention is a system comprising a simpleamplifier which has negative or gain-reducing feedbacks that reducedistortion and render theinsertion gain of the amplifier independent offrequency.

The feedbacks may be, for example, a shunt-- series negative feedbackand a series-shunt negative feedback. (A' shunt-series feedback hererefer's'to a feedbackthat is shunt feedback at the amplifier input andseries feedback at the amplifier output; a series-shunt feedback refersto a feedback that is series feedback at the amplifier input and shuntfeedback at the amplifier output) The shunt-series feedback may beobtained,

for example, by a feedback coils in series in the output circuit andinductively related to the windings of an input transformer for theamplifier, and the series-shunt feedback may be obtained, for example,by a feedback coil in series in the input circuit and inductivelyrelated to the windings of an output transformer for the amplifier.

If desired, with considerable amounts of negative feedback of both.types (1. e., shu.nt-series and series-shunt) the insertion gain of theamplifier can be made independent of frequency and the amplifier. can!be transparent as to impedance, i. e., can have its input and outputim-. pedances ZA and ZB determined by and equal to the attachedimpedances Z2 and Z1, respectively, of the outgoing and incoming linesor circuits. The transparency is a very useful property, for example,where it is desired to insert amplification in a circuit such as afilter or a long trans- 'mission line without disturbing the impedanceconditions. This is especially desirable in the case of resistance orinductance loaded transmission circuits, where, ordinarily, when anamplifier that is not transparent is inserted, the insertion must beeffected at particular points in the loading section or the loadingsection adjusted to take care of the position of the repeater.

If desired. the feedback coil in series in the output circuit of theamplifier may be connected in' the secondary circuit of the output transformer; and if desired, the feedback coil in series in the input circutmay be connected in the pri mary circuit of the input transformer. Thefeedbacks then more effectively reduce distortion in the transformers,as, for example, distortion caused by modulation in the transformers orby variation of the transmission efficiency of the transformers withfrequency; yet the primary winding of'the input transformer and thesecondarywinding. of the output transformer need not be connectedotherwise than electromagnetically, and if desired, the incoming andoutgoing lines may be balanced with respect to ground.

Other objects and aspects of the invention will be apparent fromthefollowing'description and claims.,: h

Fig. 1 is a circuitdiagram of a wave amplifying system embodyinga formof the invention employing shunt-series and series-shunt feedkslz tFigs; 1A andlB'show modifications-of the system of'Fig. 1;

Fig. '2 is a' circuit diagram of another wave amplifying systemembodyinga form of the invention employing shunt-series and series-shuntfeedbacks; and

Fig. 2A shows a modification of the system of Fig.2.

The system of Fig. 1 comprises an amplifier whose amplifying path maybe, for example, an amplifyingelemerit l of the vacuum tube type havinga single vacuum tube stage or any desired number .of tandemconnectedstages, G and P- designating the grid of the first tube and the plate ofthe last tube.

The amplifier may be, for example, a stabilized feedback amplifier ofthe general type in which a portion of the output wave is fed back ingainreducing phase and in amount sufiicient to reduce distortion belowthe distortion level without feedback. Such feedback is disclosed, foreX- ample; in HS. Black Patent No. 2,102,671, De-

cember,21,j1 937, for wave translation system, and I Waves to beamplified by the amplifier, as, for example, speech current waves orspeech modulated carrier waves, are supplied from an incoming line orcircuit Z1 of impedance value Z1 comprising a wave source E of voltageE. The amplified waves are transmitted to outgoing line or circuit 22 ofimpedance value Z2.

The amplifier comprises two feedback paths or circuits 6 and 1. Path 6comprises a feedback coil m2 of mg active turns on transformer 2. Thiscoil m2 is in series with winding 11.1, and provides shunt-seriesnegative feedback. Any parasitic series-series feedback which itproduces does not materially affect the operation of the circuitprovided such feedback is small compared to the shunt-series andseries-shunt feedbacks. Path 1 comprises a feedback coil m of m activeturns on transformer 5. This coil n2 is in series with winding m3, andprovides series-shunt negative feedback. A C-battery B for providingnegative grid bias is shown in the lead from the lower end of coil M. AB-battery 9 is shown for supplying space current. If desired, anyseriesseries feedback produced by the impedance of coil m2 can beavoided by shifting the connection of the positive terminal of battery 8to the cathode.

The amplifying path of the amplifier referred to as the ,u-circuit, andthe feedback circuits may be referred to as the B-circuits, or theSi-circuit 6 and the biz-circuit 1, the significance of ,u and B beingas indicated in the patent and article mentioned above. The feedbackthrough path B may be, for example, negative feedback with m81 1 and thefeedback through path I may be, for example, negative feedback with32 1. Preferably the coupling between 1212 and mi, and also the couplingbetween 11.2 and n3, is as close as practicable. coupling, the morenearly complete will be the correction that the feedback produces inthe' transformer distortion (e. g., the modulation produced in thetransformers and the distortion due to variation of the transmissionefiiciency of the transformers with frequency);

may be The closer this It can be shown that the input impedance withfeedback is n; 2 n oi- 45 i-l hway] where R0 is the plate impedance inthe last tube. .With considerable amounts of feedback, or in other wordsif a be made large, so that all terms involving ,u are large compared toall other terms in the equation, the equation may be simplified and theinput impedance becomes:

(2) 1 l fi ace m m Then if the expression involving the turns ratios bemade equal to ,unity,

It can also be shown that the output impedance is:

(Ll-A) The factor involving the turns ratios is the inverse of that in(l-A) and is unity when that in (l-A) is unity, making V The voltageamplification, which is the ratio of the voltage V across Z2 to thevoltage E can be shown to be becomes Z2, and if the amplifier input ister-' minated in Z1 the value ZB of the amplifier output impedancebecomes. Z1. As indicated above, this transparency as to impedance is avery useful property.

With an amplifier of any number of stages and input and outputtransformers with three windings as shown, the gain and input and outputimpedances may be made to have the values indi-.

cated by the following formulae.

2 2 I2 Ii 2 m 3 In2 +[(m (n.)

If the feedback is large (3) simplifies to 121 m 11 v EX?) E 2 I11 2 m22 3 Here again under the conditions of the turns ratios above,

The transmission is then times the transmission'before the amplifier wasinserted. Thus, the insertion gain ,of the amplifier is then constant orindependent of frequency. V

similar properties are Obtained. with the circuit 3 former distortion(as for example the modulation and the distortion due to variation ofthe transmission efliciency of the transformers with frequency) as wellas the variation of the impedances of the transformers with frequency ismore effectively reduced by thefeedback, so ZA, Zn and aremore directfunctions oftheturns ratios.

It can be shown that, with 'a'large amount of This circuit is welladapted for use in a resistance loaded cable system, for example. Anadvantageous feature is that the amplifier may be placed at any point onthe cable. Moreover, the system can be modified as indicated in Fig. 13,so that direct current, for use in heating the filament of amplifier I,for example, can readily be simplexed over the cable and. passed aroundthe amplifier without disturbing the feedback conditions in thetransmission path.

In Fig. 1B, with switches l6, l1 and I2 in the condition shown, anA-battery vl3 supplies heating current for the amplifier filament.However, instead, current for heating the filament can be supplied frombattery l4 over the line or cable L in which the amplifier is connected.This can be done by closing switches I2, closing switches H on theirupper contacts, and closing switches ID on their outer contacts. Coilsl5 and I6, through which the direct current from battery I4 is simplexedover the line L, are balanced to ground; as are the line windings m1 andn and feedback windings m2 and m of the transformers 2' and 5' whichcorrespond to transformers 2 and 5 of Fig. 1A. The centers of thefeedback windings m2 and 122 can be tied together through switches l Iand I0 and the amplifier filament, for supplying filament heatingcurrent from battery M as just described, or can be grounded together atswitches II with switches I2 thenopen and current for heating theamplifier filament then supplied by battery l3 through switches [0closed on their inner contacts.

The amplifier is bilaterally transmitting. It can pass waves to betransmitted from Z2 to Z1, for example, where two-way transmissionthrough the amplifier is desired. However, where it has a gain in onedirection, it introduces an equal loss in the other direction.

The properties of the circuit of Fig. 1A pointed out above can beobtained without the auxiliary windings on the transformers, byemploying resistance connections, for example as shown in Fig. 2. Inthis figure a resistance 58 is included in series with Z1, and aresistance 59 is included in series with Z2. Conductors 6| and 62 form afeedback path P1 providing shunt-series negative feedback; andconductors 62 and 63 form a feedback path P2 providing series-shuntnegative feedback. Resistances 64, 65 and 66 are included in conductors6|, 62 and 63, respectively. These resistances, as well as theresistances 58 and 59, are shown as variable. These resistances may bevaried to adjust the gain and input and output impedances' of theamplifier. For instance, if the feedback is large and resistances 58 and59 are small compared to Z1 and Z2 respectively and resistances 64, 65and 66 large compared to both Z1 and Z2, it can be shown thattransparency may be realized when resistance 58 equals 59 and resistance64 equals 66. The insertion gain between Z1 and Z2 will then be aconstant equal to the sum of resistances 64 and 65 divided by resistance58.

With large amounts of negative feedback this circuit gives substantiallycomplete correction of transmission variations in the amplifier andtransform-er 2 and 3.

Fig. 2a shows a modification of Fig. 2 in which a circuit balanced withrespect to ground is obtained by placing the feedback connectionssymmetrical With respect to the line impedances. A resistance 1' isconnected in series with winding 1121, between the two halves of thewinding, and a similar resistance r is connected in series with windingn3, between the two halves of the winding. Conductors H and 12 form afeedback path F1 providing shunt-series negative feedback; andconductors 13 and 14 form a feedback path F2 providing series-shuntnegative feedback. Resistances R are included in the respectiveconductors H, 12, 13 and 14. These resistances, as well as theresistances r, are shown as variable.

.It can be shown that, when 1' is small compared to Z1 and Z2 and R.large compared to Z1 and Z2, transparency may be obtained when thefeedback is large. Also under thesame conditions the insertion gainbetween Z1 and Z2 will be a constant equal to The conductors 1|, 12, 13and 14 form,'in addition to' the feedback paths F1 and F2, fourincidental feedback paths each producing negative feedback. One of thesefour incidental paths is formed by conductors H and 13, and producesshunt-shunt negative feedback. A second is formed by conductors l4 and12, and produces shunt-shunt negative feedback. A third is formed byconductors H and 74, and produces (shunt and series)-(shunt and series)negative feedback. The fourth is formed by conductors I3 and I2, andproduces (shunt and series)- (shunt and series) negative feedback.

These paths, while they exist, are only parasitic and do not modify theresults when the feedback through the path F1 and F2 is large.

'What is claimed is:

1. A wave amplifying device having input and output circuits, a Wavesource for association with said input circuit, an input transformerhaving a 5 primary winding connected to said source and a secondarywinding connected to said input circuit, a load circuit for said device,an output transformer having a primary winding connected to said outputcircuit and a secondary winding connected to said load circuit,'meansfor producing negative feedback in said device through thefirstmentioned primary and secondary windings, said means comprising animpedance in serial relation to said first-mentioned primary windingwith respect to said wave source and effectively in shunt relation tosaid load circuit with respect to said output circuit, and means forproducing negative feedback in said device of voltage induced in saidsecond-mentioned secondary Winding by current from said output circuitflowing in said secondmentioned primary winding, said latter meanscomprising an impedance in serial relation to said load circuit withrespect to said second-mentioned secondary winding and effectively inshunt relation'to said input circuit with respect to said source.

2. A wave transmission system comprising a wave amplifying device havinginput and output circuits, a wave source, an input transformer forassociating said source with said input circuit, a load circuit, anoutput transformer for associating said output circuit with said loadcircuit, and means transmitting waves from said output circuit to saidsource and producing gain-reducing feedback in said device through eachof said transformers, said means comprising an impedance connectedeffectively in shunt relation to said input circuit with respect to saidsource and connected in series with said secondary winding ofsaid-output transformer and said load circuit, and an impedanceconnected in series with said primary winding of said input transformerand said source and connected effectively in shunt relation to said loadcircuit with respect to said output circuit.

STANLEY T. MEYERS.

